Numerous methods for producing porous plastics have been described in prior patents and other publications and generally these methods either are uneconomical, do not work well, or ignore ecological considerations. For example, U.S. Pat. No. 3,378,507 discloses the use of octyl sulfosuccinate milled into polyethylene in large amounts and extracted by water to yield a porous structure. With this method, it has been found that large amounts of the expensive sulfosuccinate are needed, on the order of 100 parts by weight per 100 parts of resin (PHR) and even in such quantities only relatively low extraction percentages are obtained indicative of low porosity.
A copending U.S. Pat. application Ser. No. 160,573, filed July 7, 1971, in which Applicants herein are coinventors, discloses the use of polyvinyl methyl ether together with surface-active materials to obtain microporous reverse-osmosis membrane supports. These materials were compounded with polyvinyl chloride in amounts of about 17-40 volume percent and are leached out by water to form very fine pores or channels (0.2 - 0.4 micron), too fine for most purposes except reverse-osmosis membrane supports. Moreover, the water flow rates through such fine pore membrane supports are much lower than in porous plastics produced by the method herein disclosed. In addition, it was found that suitable porosity by this method was obtainable only in relatively thin sheets on the order of 0.010 inch and that attempts to utilize this procedure in sheets of substantially greater thicknesses have not proved efficacious.
The use of solid particulate matter for making porous plastics has also been disclosed in a number of patents. U.S. Pat. No. 3,376,238 discloses milling sugar into plastic and extracting with a solvent, such as sulfuric acid. U.S. Pat. Nos. 2,984,869 and 2,819,981 relate to the use of salt in a plastisol which is fused. The salt is purportedly leached out with water, but this process has not proved satisfactory, since it does not produce a channel network which extends entirely through the plastic body. Consequently, for most purposes, inadequate porosity is the result of using soluble particulate matter, such as salt.
U.S. Pat. Nos. 3,378,507 and 3,379,658 relate to the use of a sodium borate hydrate, together with salt crystals in a plastisol. The purpose of the borate hydrate is to give off steam during fusion of the plastisol and thereby to promote vapor pockets intended to provide inter-crystal channeling.
U.S. Pat. No. 2,819,981 relates to improving the "hand" and "slip" characteristics of flexible, vapor permeable plasticized polyvinyl chloride films by the use of a softener as a replacement or in addition to the plasticizer. Mention is made that the softener can replace part of the pore-forming material. Porosity is not enhanced, however, as indicated in terms of water vapor transmission by the addition of such softeners and in thicker coatings the vapor transmission tends to be reduced.
U.S. Pat. No. 3,536,796 relates to the use, in a very high molecular weight polyolefin having a melt index of 0, of at least 30 volume percent of a plasticizer together with filler particles to make porous plastic sheet material. This process is limited to one polymer, is extremely costly because it involves the use of such large quantities of plasticizer and relates to the use of multiple leaching steps using organic solvents.
U.S. Pat. No. 3,551,538 teaches the use of polyethylene oxide in thermoplastic but only about 4-12% of the water-soluble material leaches out and the method is thus only suitable for paper-like structures.
It is the principal object of this invention to provide an improved method of making porous plastics using a combination of plastic-insoluble, water soluble particulate matter in an amount of at least about 45 volume percent and channel forming liquids.
It is a further object of this invention to provide an improved method wherein the solid and liquid materials to be leached from the plastic are relatively inexpensive, and wherein the method utilizes simple water extraction techniques and solid particulate materials, the extraction and disposal of which does not damage the ecology. Indeed, some of the extracted salts can be used as fertilizers and moreover, the invention provides for the use of minimal amounts of channeling liquid which can be readily removed and recycled in the processing.
Another object of this invention is to provide a more efficient method of making porous plastics of superior porosity not heretofore available by water leaching techniques using a combination of water soluble particulate solid and one or more water soluble liquids having the characteristic of forming channels between the voids formed by the solid particles after they have been dissolved.
The above and other objects and advantages of this invention will be more readily appreciated from the following detailed description.
The method embodying this invention may be applied to the production of porous plastics intended for any use, including among others artificial leather, inking pads, ion exchange resins, porous electrodes, porous bearings, filtration devices, fibers and papers, battery separators, vent devices, scrim and textile stiffening devices, aeration and emulsification devices, levitation devices, slow leaching devices, sachet type devices, evaporation devices, surgical implants, embossing, enzyme or catalyst supports, porous carbon structures, adhesive bonding aid, bandages and blood aeration devices, accoustical foam, high fiber content compositions, Kieselguhr substitutes, gasketing and air permeable wall panels.
The method embodying this invention involves the blending with a suitable plastic mass of a plastic-insoluble, water soluble liquid and a water soluble foreign solid particulate matter, such as salt in a critical amount which will be referred to as "critical volume loading" and a water soluble channel forming liquid. Effective mixing of these ingredients may be accomplished in any suitable manner, such as by the use of a Banbury mixer or a twin screw extruder, which has been found particularly effective in such mixing. The compounded material can then be calendered, extruded or compression molded. After the plastic mixture is formed into the desired plastic body, sheet or film structure, its water soluble and liquid fractions may be leached out by immersion of the plastic body into a fresh water bath. The water bath may be suitably agitated to effect proper leaching of the soluble additives. The effectiveness of this process has been determined by comparing the weights of the plastic body before and after leaching and calculating the percentage by weight of the water soluble fraction leached from the plastic. Invariably, the percentages obtained using the process embodying this invention are 80% or greater and generally range in the neighborhood of 90-99%. In contrast, when using either liquid or particulate solids alone, the percentages were generally substantially lower then 80%, particularly in specimens of comparable thickness.
Polymers suitable for purposes of this invention include materials which are fluid at some stage in their processing and which are substantially non-solvents for the foreign solid and the liquid to be subsequently leached by water immersion. Suitable thermoplastics for carrying out this invention include: unplasticized polyvinyl chloride, polyvinyl chloride-propylene copolymer, polyvinyl chloride-ethylene copolymers, polyvinylidene chloride copolymers, polystyrene, impact styrene, ABS resin, styrene butadiene block copolymers, polyethylene low (0.91 sp. gr.) to high density (0.965 sp. gr.), polyethylene copolymers with propylene, butene, 1-pentane, 1-octane, hexene, styrene, etc., polyethylene copolymers with vinyl acetate, alkyl acrylate, sodium acrylate, acrylic acid; etc., chlorinated polyethylene, chlorosulfonated polyethylene, polypropylene and propylene-olefin copolymers, polybutene and butylene-olefin copolymers, poly 4-methyl 1-pentene, thermoplastic polyurethanes, polyamides, e.g. Nylon 6, Nylon 12, Nylon 6/6, Nylon 6/10, Nylon 11, fluorocarbon resins such as FEP, polyvinylidene fluoride, polychlorotrifluoroethylene; acrylonitrile - methyl acrylate copolymers, acrylonitrile - vinyl chloride copolymers, methacrylonitrile-styrene copolymers, polymethyl methacrylate, cellulose acetate, cellulose acetate butyrate, acetal, polycarbonate, polysulfone, polyphenylene oxide, polyethylene and butylene terephthates.
Many thermosetting resins and crosslinkable resins are also suitable for purposes of this invention and include the following: radiation cured polyethylene, peroxide-cured polyethylene, diazo crosslinked polypropylene, epoxy resins; hydrocarbon, chloroprene, and nitrile rubbers, furane, melamine-formaldehyde, urea-formaldehyde, phenol-formaldehyde, diallyl phthalate, polyesters and silicones. Extraction of the soluble constituents may be accomplished before or after crosslinking depending upon the polymer selected. Gums, such as gelatine, collagen, carboxymethyl cellulose, polyacrylamide and polysaccharide may also be used provided they can be rendered water insoluble before extraction. Extraction of the soluble constituents is carried out for such water insolubilizable gums after crosslinking.
In accordance with this invention the solids constituent is used in an amount of at least about 45 volume percent of the total blend which herein is defined as "critical volume loading". It has been found that finely divided particulate materials are suitable for use in carrying out this invention provided the material selected is soluble in water, preferably at room temperatures or decomposes with minimum residue and does not melt or decompose at the polymer compounding temperature. It is, of course, also essential that the solid material selected be insoluble in the plastic mass with which it is used and be compatible therewith. Suitable materials include: inorganic salts and acids, organic salts and organic solids, such as for example: sodium chloride, sodium borate, sodium carbonate, potassium chloride, ammonium chloride, ammonium carbonate, calcium chloride, boric acid, sodium acetate, sodium formate, urea, thiourea, saccharine, starch and lactose. The particle size of the foreign solid, for most purposes, should preferably be about 0.001 inch in diameter. Since particles in this range will leave cavities which are not readily visible to the naked eye, flexible sheets made by this process would be suitable for poromeric usage. For other purposes, including: shoe interiors, coarse filtration membranes, aeration and levitation devices, larger particle size solids may be utilized, such as on the order of 0.010 - 0.020 inch. Of course, where finer channels are desired, such as in certain types of filtration, plastic paper or poroous fiber, fuel cell and battery separators, a finer particle could be utilized, such as 1-5 microns.
Channeling liquids which may be used in carrying out the method embodying this invention include essentially any liquid (having a viscosity below 10,000 poise at processing temperature), or combination of liquids, which is substantially insoluble in and compatible with both the polymer to be made porous and the particulate solid material used. The liquid must be capable of forming channels interconnecting the solid particles dispersed polymer and be readily water soluble so that it can be economically leached together with the particulate solid fraction. Thus is provided a pore structure which includes the cavities left by the dissolved crystals and the interconnecting channels formed between the crystals by the liquid. At "critical volume loading" of the dispersed particles surprisingly small quantities of liquid will yield this channel network. Examples of suitable channel forming liquid include detergents or surfactants and various types of water-soluble oils including glycols and polymeric oils. Polyvinyl methyl ether having a molecular weight varying from 500-500,000, polyethylene oxide having a molecular weight varying from 1,000-1,000,000 are suitable polymeric oils. Suitable glycols, detergents or surface active agents may also be employed. At and above "critical volume loading", various channeling liquids in very small quantities from about 0.1 volume percent may be used, including the polymeric oils listed above, glycols and various non-ionic, anionic and cationic surface active agents, including for example:
Useful non-ionic detergents or surfactants based on ethylene oxide condensate derivatives including but not limited by: PA0 Useful anionic detergent types include but are not limited by: PA0 Useful cationic and amphoteric detergents include but are not limited to:
__________________________________________________________________________ Hydrophobic Base Commercial Equivalents HLB No. __________________________________________________________________________ alkyl aryl phenols e.g. Rohm & Haas Triton X-100 13.5 (t-octyl phenol with 9-10 ethylene oxide units) GAF Igepal CO880 (nonylphenol 17.2 with 30 ethylene oxide units) fatty sorbose e.g. Atlas Tween 20 (polyoxy- 16.7 ethylene sorbitan monolaurate) fatty alcohols e.g. Atlas Brij 35 polyoxyethylene 16.9 lauryl ether fatty mercaptides e.g. Pennsalt Nonic 218 (poly- -- ethylene glycol tertdodecylthio ether fatty acids e.g. Atlas Myrj 52 (polyoxyethylene, 16.9 40 units, monostearate) Atlas G-1790 (polyoxyethylene 11 lanolin) vegetable oils e.g. Atlas G-1794 (polyoxyethylene 13.3 castor oil) silicones e.g. Union Carbide L-77 -- (dimethyl silicone-polyalkylene oxide) acetylenic e.g. Airco Surfynol 475 (ethylene 15 alcohols oxide condensate, 75% weight, with 2, 4, 7, 9-tetramethyl-5- decyne-4, 7-diol) __________________________________________________________________________
Low foam detergents are now available and can be utilized. These materials frequently consist of one of the above categories (usually polyoxyethylene alkyl aryl phenols) with the oxyethylene terminated by a methyl or benzyl group instead of hydrogen. Detergents based on ethylene oxide-propylene oxide-ethylene oxide block polymers are also useful; e.g. Wyandotte Pluronic F68 (80% ethylene oxide block polymer on both sides of a 1750 MW propylene oxide central unit). The HLB No. of the specific examples given, last column above, is defined by Atlas Chemical Co. as: HLB = (E + P)/5 (Griffin, Journal of the Society of Cosmetic Chemists, pg. 251, vol. V, No. 4, Dec. 1954)
wherein HLB + Hydrophile/Lipophile balance
E = weight percentage of oxyethylene content PA1 P = weight percentage of polyhydric alcohol content (glycerol, sorbitol, etc.) PA1 Glycerine: (particularly for medical applications) PA1 Emcol PG; Witco Polymerized glycerol PA1 Ethylene glycol PA1 Polyethylene glycol (MW 136-1000) PA1 Pentaerythritol -- where heated processing is utilized PA1 Water -- where the processing temperature without pressure does not exceed 95.degree. C.
We have not found any particular water soluble or dispersible non-ionic to be unsuitable to the purposes of this invention. Water soluble detergents are preferable to "dispersible" detergents, however. By "dispersible" we mean the ability of the solid or liquid detergent to form a hazy sol in water at room temperature resulting in the disappearance for all practical purposes of the original solid or oil. We cannot define insolubility vs. dispersibility on the basis of HLB number, however, since there is no clear cut-off point. For example, Baker Castor Oil Co. Surfactol 318 castor oil-polyethylene oxide (HLB 3.6) is water dispersible while Atlas Tween 65 polyoxyethylene sorbitan tristearate (HLB 10.5) is water insoluble. We prefer to use detergents containing straight chain hydrocarbons because such materials are more readily biodegradable thereby simplifying disposal problems after the leaching operation.
With regard to ionic detergents, any water soluble or dispersible compound which will withstand the rigors of milling or other melt mastication with the molten thermoplastic appears to be suitable.
The HLB equivalent of ionic detergents and soaps is very high; i.e., 18-40, hence almost universal water solubility is not surprising.
__________________________________________________________________________ Alkyl aryl sulfonates e.g. sodium dodecyldiphenyl ether disulfonate, dodecyl benzene sodium sulfonate, butyl benzene sodium sulfonate Alkyl sulfate e.g. sodium lauryl sulfate Alkyl sulfonate e.g. sodium ethyl cyclohexane p-sulfonate, castor oil sulfonate (turkey red oil) Napthalene sulfonate e.g. sodium isopropyl napthalene sulfonate, sodium tetrahydro napthalene sulfonate Phenol sulfonate e.g. sodium butyl phenyl phenol sulfonate Sulfonated ethoxylated e.g. sodium dodecyloxyethyl sulfonate alkyl alcohol Sulfonated alkyl aryl e.g. Rohm & Haas Triton X200 (sodium polyether salt of alkyl aryl polyether sulfonate) Sulfonated esters e.g. sodium di(2-ethyl hexyl) sulfosuccinate, sodium diamyl sulfosuccinate Alkyl or alkyl aryl e.g. Rohm & Haas Surfactant QS-30 phosphate esters Witco ("Emcol" or GAF "Gafac" alkyl or alkyl aryl polyethylene oxide phosphates, Lecithin Fluorocarbon phosphate e.g. ##STR1## Fatty or vegetable acid e.g. sodium oleate trithanol amine stearate __________________________________________________________________________
Cations other than sodium; i.e., potassium, lithium, hydrogen, morpholine, ammonium, triethanol amine, etc., may be utilized with the detergent anion. Multivalent cations such as calcium, magnesium, and barium are sometimes water insoluble, especially with carboxylates, and hence frequently not of interest. Many multivalent-cation-sulfate and sulfonate detergents are water soluble, however (e.g. calcium, magnesium, zinc and/or barium dodecyl diphenyl ether disulfonate salts are substantially water soluble). We prefer sodium because of cost and convenience. The hydrogen or acid form of the detergent is sometimes undesirable from the point of view of equipment corrosion. Ammonium, morpholine and volatile amines can be utilized but the volatility makes them less desirable than the nonvolatile metal salts.
Alkyl amines and ethanol amines such as tributyl amine or triethanol amine are less volatile. Higher molecular weight alkylated amines such as trinonyl amine are usually less desirable because solubility in the plastic phase and water insolubility of the detergent frequently occurs at this point of substitution.
__________________________________________________________________________ Amine oxide e.g. Armour Armox C/12 bis(2 hydroxy ethyl) cocoamine oxide Ethoxylated amine e.g. Armour phenyl stearyl amine (ethoxylate 15 mol) Ethoxylated amide e.g. Nopco AO-43 Polyoxyethylene oleic amide Quaternary e.g. Cetyl trimethyl ammonium chloride Amphoteric e.g. General Mills Deriphat 170C N-lauryl amino propionic acid, Geigy Sarkosyl L lauroyl sarcosine __________________________________________________________________________
Hence a wide variety of surfactants are suitable. The only requirement being solubility or dispersibility in water. Detergents with an aromatic ring in their hydrophobic portion are preferred, however. Apparently the detergent coats the foreign solid and by capillarity causes a bridging action between very close, barely touching or touching particles. Towards this end, we have found that a minimum of 0.1 volume percent detergent can be utilized at approximately 45-79.9 volume percent foreign solid content.
Minimal amounts of DuPont Duponol ME and General Mills Deriphat 170C should be utilized in polyvinyl chloride because these detergents cause heat stability problems.
The glycol adjuvants consist of low molecular weight polyhydroxy materials limited to the following:
Water is generally not used in large amounts but can be utilized in fairly large amounts if it is saturated with the foreign solid before use.
It has also been found that water itself may be used for certain plastics where the water, if used in larger amounts, is saturated with the foreign solids before blending into the polymer.
The amount and selection of channeling liquid depends upon a number of factors, including the type of plastic which is being made into a porous material and the amount of particulate solid matter utilized whether above or below about 45 volume percent loading. At or above this "critical volume loading" an almost unlimited variety of channeling liquids are suitable for use, numerous examples of which are listed above. A sufficient amount of channeling liquid must be insoluble in the main body of the plastic so that the liquid channels can be established between the crystals of the solid particulate matter employed. If the viscosity of the channeling liquid is too great, i.e., over 10,000 poise, very large diameter channels will result in consequent poor cohesion of the plastic mass during compounding and incomplete interconnection of the salt crystals.
In carrying out the process embodying this invention, a polymer such as a molding powder, preferably in the amount of 20-55 volume percent, may be blended with a suitable water-soluble or heat decomposible particulate solid preferably in the amount of about 45-79.9 volume percent and a channeling liquid in the amount of 0.1-15 volume percent; antioxidants or other stabilizers may also be added. To assist in processing certain thermoplastics, it is sometimes desirable to control melt flow by the use of at least 3-4% channeling liquid and sometimes 10-20% liquid at very high salt loadings. The polymer phase may include polymer per se and up to about 80% by volume (preferably 0-50%) of a suitable filler material, such as clay, chrysotile and crocidolite, asbestos fibers and glass fibers. If the PVME utilized is a water solution, the water flashes off during the compounding at temperatures over 95.degree. C. Nylon and urethane compounding may be carried out in a Banbury type mixer or extruder at 200.degree.-280.degree. C because of the high fluxing temperature requirements. Solid matter may be added and worked in after the water-soluble oils are added. Powdered polyethylene and polyvinyl chloride suspension resins are preferably mixed at 80.degree. C with the particulate solid and channeling liquid in a mixer such as a Prodex type mixer. The mixture is then dried and extruded in a vented extruder to remove residual water. A twin screw extruder provides effective means for this processing.
The compounded material can then be calendered, extruded or compression molded. Properly compounded stock up to one-eighth inch in thickness can be satisfactorily leached by tumbling 8-16 hours in water or by immersing the molded object in an agitated tank with fresh water for 8-16 hours. Compounded sheets one-fourth inch thick can be rendered porous by leaching in a tank for 4 hours and then forcing water through the sheets while held in a suitable frame type fixture. The leaching rate of thin sheets can be greatly accelerated by the use of ultrasonic agitation which speeds solution and diffusion of the additives. In most instances hot water affords a faster leaching rate than cold water.